The medical device industry produces a wide variety of electronic devices for treating patient medical conditions. Depending upon the medical condition, medical devices can be surgically implanted or connected externally to the patient receiving treatment. For example, implantable neurostimulators are available for the treatment of pain, movement disorders such as Parkinson's disease, essential tremor, dystonia, gastric disorders, incontinence, sexual disfunction, migraine headaches, and other conditions. Other examples of IMDs include, but are not limited to, implantable drug infusion pumps, cardioverters, cardiac pacemakers, defibrillators, and cochlear implants.
Because IMDs provide important, oftentimes life-sustaining, medical care to patients from the power supplied by a single component power source, usually a battery, the ability to know the status of that power source is critical. When a power source's charge has nearly run down, the power source must either be replaced or recharged. The failure to do so could result in the untimely failure of the IMD's ability to deliver therapy to the patient.
Since it is often critical for patients' well-being that IMDs do not cease operating, it is common for IMDs to monitor the level of battery depletion and to provide some indication when the depletion reaches a level at which the battery should be replaced. In the case of IMDs employing a non-rechargeable battery, it is typical for the IMD to monitor battery energy and depletion and develop an “Elective Replacement Indicator” (ERI). This ERI indicates when the battery depletion reaches a level such that replacement will soon be needed.
Once the ERI indicator has been activated, circuitry in the IMD may respond by switching or deactivating operating modes to lower power consumption. This will prolong the interval between ERI and the time when the battery is completely depleted, a condition referred to as battery End-of-Life (EOL). For example, internal diagnostic functions and advanced rate-response functions may be discontinued upon issuance of ERI. Additionally, in the case of IMDs that are delivering electrical stimulation pulses, frequency or amplitude of the delivered pulses may be decreased to conserve energy. This will allow the IMD to continue to operate for at least some minimum amount of time after issuance of an ERI. In this way, the physician will have sufficient time to take appropriate action. For an IMD employing a non-rechargeable power source, this will involve replacing the device before battery EOL.
While it is important to indicate ERI soon enough to allow for replacement of the device, it is also important not to trigger ERI too early or due to transient faults. If ERI is indicated too early, sudden operational changes associated with ERI may be made before such modifications are actually necessary. Thus, it is desirable to have an accurate estimate of the battery capacity that has been used and/or the battery capacity that remains to be used so that ERI is as accurate as possible.
In devices that employ a rechargeable power source, there is no need to replace the device when the battery is depleted. Instead, the power source is merely recharged via transcutaneous transmission of energy. In this case, an external power supply is operatively coupled with the rechargeable power source of the implantable medical device, often through an inductive link. Charging current is delivered to the rechargeable power source until the rechargeable power source is replenished. The implantable medical device may then continue to deliver therapy to the patient until the rechargeable power source has again been depleted and the process is repeated.
Even in the case of a rechargeable power source, obtaining an accurate determination of the amount of charge remaining on the battery is desirable. This will allow the patient to more conveniently re-schedule the recharge session so that it may occur before battery charge has been depleted.
One mechanism for estimating the battery capacity that has been used involves measuring either open-circuit battery voltage or battery impedance. In either case, the battery must be temporarily disconnected from the circuit to which the battery supplies power. After obtaining the open-circuit voltage or impedance measurement is this manner, the measurement is compared to battery voltage depletion or impedance characteristics, respectively. This comparison provides an estimate of the battery capacity that has been used, as well as the charge remaining on the battery, and hence the time remaining before battery recharge or replacement must be initiated.
As discussed above, measuring open-circuit battery voltage or impedance within an IMD using traditional methods has required the battery to be temporarily disconnected from the circuit to which the battery supplies power. When this occurs, some alternative method of supplying power to at least the portion of the circuitry that is obtained the measurement will be required. This may involve providing a capacitor or another alternative power supply. This increases the size and complexity of the circuit design in most cases. Another mechanism for monitoring battery capacity is therefore desired.